There have conventionally been used a magnetic drum, magnetic tape, magnetic disc and the like as a magnetic recording medium for a computer. These mediums have some advantages and some disadvantages respectively and are individually selected for their most suitable uses. Totally the demand for magnetic discs is now largest in the industry in consideration of the operative speed, recording capacity, mounting space, etc. However, in accordance with recent development of microcomputers excellent in function and significantly miniaturized for portable use, the time has come when the industry must reconsider the situation.
FIG. 6 shows the recording capacities of an existing magnetic disc and magnetic drum under fixed conditions, i.e. with 41.5 millimeters thick apparatuses, 300TPI and 10kBPI. The ordinate represents the recording capacity MB, and the abscissa indicates the diameter of the recording medium in inch.
Lines A and B show theoretical recording capacities of discs for single-surface use (line A) and two-surfaces use (line B). In practice, however, the expansion coefficient of the disc, the diameter of the spindle shaft and other material limit the recording capacities to the value shown by line C.
Drums have the recording capacities shown by dotted line D. In larger diameter ranges, discs provide larger recording capacities than drums. However, as the diameter decreases, the difference in recording capacity between discs and drums decreases, and drums provide larger capacities in 3.5 inches and less diameter ranges. This means that magnetic drums are suitable for use in compact systems where the diameter of the recording medium must be small.
These circumstances are further explained in detail, referring to FIGS. 7 and 8. FIG. 7 at A and B schematically shows a two-surfaces-recording magnetic disc 1 with two magnetic heads 2--2 coplanarly, radially movable with respect to the disc 1. FIG. 8 at A and B schematically shows a magnetic drum 3 with a single magnetic head 4 movable axially along the outer peripheral surface of the drum 3.
Referring to FIG. 7, since the medium (disc 1) is rotated at a constant speed, the signal density changes from a large diameter turn and a small diameter turn, and becomes largest in the innermost turn. Since the recording capacity in each turn of a disc must be the same as the capacity of the innermost turn, the total recording density decreases as the diameter of the disc 1 increases. Additionally, the output magnitude of the head 2 changes proportionally to the relative movement velocity between the disc 1 and the head 2, and significantly decreases in the innermost turn where the relative velocity is smallest.
In view of the limitation to the number of tracks and the decrease of the output magnitude, the minimum effective diameter of a disc will be about 3.5 inches, and no disc cannot be used in an apparatus where a space of 2.5 inches or less is simply allowed for the diameter.
These problems involved in magnetic discs do not arise in magnetic drums because the magnetic drum 3 as shown in FIG. 8 is constant in diameter throughout the axial length thereof. Assuming that the diameter of a disc is 3.5 inches, the diameter of the innermost turn of a disc will be about 2.4 inches, and the total recording capacity of the disc is a value obtained by multiplying the capacity in the 2.4 inches diameter turn and the number of tracks. In the drum, however, all tracks have a uniform diameter of 3.5 inches and a uniform recording capacity which is 1.45 times the unit capacity of the disc. Thus, the total recording capacity of the drum is a value obtained by multiplying the significantly larger unit capacity and the number of tracks.
As described, in smaller diameter ranges, drums are sufficiently competitive to discs, and particularly in diameter ranges smaller than about 5 inches, drums are superior to discs in not only recording capacity but also signal processing facility.
However, there is a problem with a conventional magnetic drum recording apparatus wherein a belt used for driving the drum invites an increased dimension of the apparatus. This invalidates the above-mentioned advantages of drums in smaller diameter ranges.
Further, the prior art magnetic drum recording apparatus is configured as schematically shown in FIG. 8 where a single head 4 is provided adjacent the outer periphery of the drum 3 for axial movement along the periphery. Therefore, it is difficult to effectively use the entire axial length of the drum 3, and the access time is relatively long. More specifically, since the recording and reproduction head 4 is supported by a carriage which has a given length in the axial direction of the drum, if the movable range of the carriage is limited within the axial length of the drum 3, the entire length of the drum cannot be used for recording. If the carriage is configured to move in excess of the entire length of the drum in order to use the entire axial length of the drum for recording, a larger-scaled mechanism for moving the carriage will be necessary, and the access time will be further increased.